1. Field of the Invention
The present invention relates to an improved process and catalyst system for the ring-opening or metathesis polymerization of cyclic olefins, such as dicyclopentadiene (DCPD). More specifically, this invention relates to an improved organo transition metal catalyst for metathesis polymerization.
2. Description of the Related Art
Cyclic olefins are subjected to ring-opening metathesis polymerization to produce thermoplastic and thermoset polymers having physical properties which make them suitable for structural and electronic applications, such as molded plastic items and electrical laminates. Such polymerizations are commonly carried out in reaction injection molding (RIM) processes, in which a metathesis catalyst and a monomer are charged to a heated mold, and polymerization of the monomer and forming of the polymer into the desired shape are carried out simultaneously in the mold.
In such RIM processes, it is important that the polymerization reaction occur rapidly and with as complete incorporation of the charged monomers as possible. For example, the presence of unreacted monomers in molded polydicyclopentadiene has been found to result in a molded part with an unpleasant odor, and less than optimum physical properties. Finding a RIM process that reacts in as short a cycle time as possible and at mold temperatures at or near room temperature is economically desirable. It is also advantageous to be able to use a less than pure monomer stream and thus avoid extensive purification of the monomer prior to polymerization.
Numerous patents and literature references relate to such polymerization in the presence of a variety of olefin metathesis catalysts. Among the more effective ring-opening polymerization catalysts are homogenous catalyst systems based on tungsten or molybdenum halides, often employed with an organotin or organoaluminum co-catalyst. Examples of such catalyst systems are disclosed by Sjardijn et al., U.S. Pat. Nos. 4,810,762 and 5,093,441, wherein phenolic tungsten halides are used with organotin hydrides. Similar catalyst systems are disclosed by Sjardijn et al. in U.S. Pat. No. 4,729,976, which have been found to be highly active in a relatively impure DCPD feed stream.
For most cyclic olefin metathesis catalyst systems, the presence of reactive materials such as water and oxygen should be avoided because they foul the polymerization process. Small amounts of reactive material may be acceptable, but water in excess of 20 parts per million (ppm) should generally be avoided. As a result, it is usually necessary to thoroughly dry the cyclo olefin monomers prior to contact with the polymerization catalyst, and to conduct the polymerization in an inert environment.
Cyclic olefins are commonly protected from oxidation and subsequent degradation by the addition of stabilizers such as catechol, e.g. 4-t-butyl catechol. Such stabilizers are necessary because oxidation of the cyclic olefin, even in small amounts, can effectively inhibit or prevent desired metathesis polymerization thereby rendering the catalyst system ineffective. However, the catechol stabilizers themselves also interfere with the reactivity of metathesis catalysts. 1,4-Hydroquinones are also suitable as oxidation stabilizers for the cyclic olefin monomer.